A demand for broadband communications has increased accompanied with recent developments in multimedia, an optical transmission system exceeding 10 G b/s is in practical use, and a further speeding-up thereof is expected. The LN optical modulator is used as a device for modulating an electric signal (microwave signal) exceeding 10 G b/s into a light signal.
In order to meet the velocity matching condition in the structure designed to thin an optical waveguide substrate, it is necessary to make the thickness of the substrate around the optical waveguide to be about 10 μm, for example. The inventors have applied for a patent of a two-stage back-groove structure with Japanese Patent Laid-Open Publication No. 2002-169133A, in order to prevent the optical mode field pattern from flattening, and to reduce the propagation loss of light generated by the influence of a surface roughness and damages due to thinning the substrate and machining the groove.
The inventors have disclosed, in WO 03/042749A, an optical waveguide device in which a flat supporting substrate and an optical waveguide substrate are adhered.